1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically but not exclusively, to coherent optical receivers.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
An optical coherent-detection scheme is capable of detecting not only the amplitude of an optical signal, but also the signal's phase. These capabilities make optical coherent detection compatible with the use of spectrally efficient modulation formats, such as quadrature-amplitude modulation and phase-shift keying (PSK) in its various forms (e.g., differential binary PSK and differential quadrature PSK). Compared to non-coherent detectors, optical coherent detectors offer relatively easy wavelength tunability, good rejection of interference from adjacent channels in wavelength-division-multiplexing (WDM) systems, linear transformation of the electromagnetic field into an electrical signal for effective application of modern digital-signal-processing techniques, and an opportunity to use polarization-division multiplexing (PDM).
An optical coherent detector usually employs a 90-degree hybrid that mixes a received optical communication signal and a local oscillator signal so that the data carried by the optical communication signal can be recovered. However, one problem with a prior-art optical coherent detector is that it typically requires a separate 90-degree hybrid for each WDM and/or PDM component of the optical communication signal. Disadvantageously, this multiplicity of constituent devices makes the use of optical coherent detection in WDM and/or PDM systems relatively expensive and causes the corresponding receivers to be relatively large and/or to exhibit relatively high optical attenuation.